Laser light scattering has been employed to study the rigidity of human and lamprey crosslinked fibrin clots, in vitro and under physiological conditions. We are correlating clot rigidity as a quantitative function of the crosslinking fibrin subunits. Investigations of the molecular structure of salmon and lamprey fibrinogens and fibrins, in particular the A Alpha-subunits are under study. SV40 transformed rabbit uterine epithelial cells have been maintained in culture for two years.